The present invention generally relates to a process for recycling polymeric materials and more particularly to a process for converting polyester into its original chemical components: ethylene glycol and terephthalic acid.
Polyester is a polymeric material made from the esterification of polybasic organic acids with polyhydric acids. Specifically, polyester is commonly manufactured by reacting terephthalic acid with ethylene glycol resulting in a compound known chemically as polyethylene terephthalate and commonly as PET. Widely known polyesters include Dacron and Mylar.
PET is currently being used as a polymeric base material in a wide variety of applications. For example, polyester is commonly used to make photographic films, X-ray films, bases for magnetic coating such as in recording tapes, beverage containers, surgical aids such as synthetic arteries, and as a fabric for making garments and other similar items. However, although polyester is very useful, waste materials containing polyester are beginning to create a waste management and disposal problem.
Currently, those skilled in the art are seeking different methods of recovering and reusing polyester contained in waste plastic products. However, recovery of polyester from waste products has been found difficult. In particular, many prior art processes are not capable of efficiently or economically recovering polyester when a significant amount of impurities and contaminants are present. Such impurities include cellulosic materials, polymeric materials and metals. As such, most attempts have been limited to mechanical recovery processes directed to specific polyester-containing materials. In these systems, the waste materials are merely washed in order to recover polyester films.
For example, U.S. Pat. No. 4,602,046 to Buser et al., discloses a method for the recovery of polyester from scrap material such as photographic film having a polyester base and at least one layer of macromolecular organic polymer. Specifically, scrap material is cut or chopped into small individual pieces or flakes and treated in a caustic alkaline solution at a solids level of at least 25% by volume and under conditions of high shear. The organic polymer coating material is removed from the polyester flakes. The polyester flakes are then separated from the polymer coating material by filtration or centrifugation, rinsed in water, and dried. The recovered polyester flakes can be used as a feed stock for making films, bottles or other polyester articles.
A method and apparatus for recovering silver and plastic from used film is also disclosed in U.S. Pat. No. 4,392,889 to Grout. In this method, the used film is first passed through a bath preferably comprising a hot caustic solution for precipitating silver layered on the film. The film then passes through a second bath of hot caustic until an adhesive sheet disposed on the film has been dissolved. Typically, the adhesive sheet is made of polyvinylidine chloride which adheres the silver to the film. After a second caustic bath, the film is dried and available for use.
A process for the recovery of clean polyester materials is disclosed in U.S. Pat. No. 3,928,253 to Thornton et al. Specifically, the process is directed to polyester photographic film, where the polyester is coated with binders, adhesives and metal compounds. In order to recover clean polyester, polyester photographic film is first wetted with an aqueous alkaline solution of an organic solvent which loosens and detaches coatings and subcoatings from the surface of the film. The polyester film is then separated from the reagent and rinsed. The reagent is then clarified and recycled and reused on other photographic film.
U.S. Pat. No. 3,652,466 to Hittel et al., discloses another process of recovering the polyester from polyester films. The coated films are cut into small pieces and treated with a caustic aqueous alkali solution to form a slurry. The slurry is fed into a classification column in which the pieces move downward countercurrent to a moving column of aqueous liquid which separates the pieces from the coating material. The pieces are removed from the bottom of the column in suspension and can thereafter be used as a source of polyester material. Further, the coating material can be removed from the top portion of the column and silver halide can be recovered in the form of silver.
Similarly, U.S. Pat. No. 3,647,422 to Wainer discloses the recovery of silver, polyester and amino acids from processed film and U.S. Pat. No. 3,873,314 to Woo et al. discloses the recovery of clean polyester materials from photographic film.
As shown above, the cited prior art methods of recovering waste polyester are generally limited to photographic films. In recycling the photographic films, silver is also recovered, thus making the processes economically viable. Mechanical recovery in non-silver containing polyester films presently lacks such economic advantages.
It has also been discovered that the prior art processes are generally further limited to processing particular types of films. Films containing higher proportionate amounts of non-polyester materials are typically much more difficult and expensive to process. For instance, many post consumer photographic films contain contaminants such as other polymeric materials in amounts up to about 50% by weight. These polymeric materials may include polyvinyl chloride, polyvinylidine chloride, acetate, polystyrene, polyethylene, and other polyolefins. Such films typically cannot be recycled and usually are discarded into landfills.
Recently the focus of recovering polyester from the waste stream has changed from mechanical washing processes to chemically converting the recovered polyester to more useful components. For instance, one current commercial process for chemically recycling polyester is methanolysis. This process is generally directed to the recycling of PET from X-ray and/or photographic film waste. The process involves the steps of: (1) sorting the film from other plastics and papers; (2) grinding the film; (3) washing the film with appropriate chemical solutions; (4) separating the film from a resulting sludge; (5) drying the film in the form of flakes; and (6) reacting the PET flakes with methanol under pressure, in order to convert it to ethylene glycol and dimethyl terephthalate (DMT). However, methanolysis is a very expensive process and can only be used with polyesters that are relatively free of contamination. In fact, many types of PET waste cannot be used due to the high impurity content.
Because of the deficiencies in the prior art, many waste products containing polyesters are not capable of being economically recycled. As such, most polyester waste products end up in landfills. In fact, millions of pounds of polyester-containing products are discarded in landfills each year. Landfill disposal is not only expensive, but is environmentally damaging.
Consequently, the prior art is generally deficient in providing an economical process for the recycling of polyester. The prior art is also deficient in providing a process for the recovery of PET from waste materials containing contaminants and impurities. Further, the prior art is generally deficient in providing a method for recycling PET from products other than photographic and X-ray films. Also, the prior art is generally deficient in providing a method of chemically recycling polyester in which the polyester is converted into more usable chemical components, namely the raw materials from which polyester is formed. Due to the increasing production of waste materials containing polyester, it would be very desirable to have an economically viable process for recycling polyester from the waste stream.